A cohesive zone model, taking full account of finite geometry changes, is used to provide a unified framework for describing the process of void nucleation from in­itial debonding through complete decohesion. A boundary value problem simulating a periodic array of rigid spherical inclusions in an isotropically hardening elastic-viscoplastic matrix is analyzed. Dimensional considerations introduce a characteristic length into the formulation and, depending on the ratio of this characteristic length to the inclusion radius, decohesion occurs either in a "ductile" or "brittle" manner. The effect of the triaxiality of the imposed stress state on nucleation is studied and the numerical results are related to the description of void nucleation within a phenomenological constitutive framework for progressively cavitating solids. 1 Introduction The nucleation of voids from inclusions and second phase particles plays a key role in limiting the ductility and toughness of plastically deforming solids, including structural metals and composites. The voids initiate either by inclusion cracking or by decohesion of the interface, but here attention is confined to consideration of void nucleation by interfacial decohesion. Theoretical descriptions of void nucleation from second phase particles have been developed based on both continuum and dislocation concepts, e.g., Brown and Stobbs (1971), Argon et al. (1975), Chang and Asaro (1978), Goods and Brown (1979), and Fisher and Gurland (1981). These models have focussed on critical conditions for separation and have not explicitly treated propagation of the debonded zone along the interface. Interface debonding problems have been treated within the context of continuum linear elasticity theory; for example, the problem of separation of a circular cylindrical in­clusion from a matrix has been solved for an interface that supports neither shearing nor tensile normal tractions (Keer et al., 1973). The growth of a void at a rigid inclusion has been analyzed by Taya and Patterson (1982), for a nonlinear viscous solid subject to overall uniaxial straining and with the strength of the interface neglected. The model introduced in this investigation is aimed at describing the evolution from initial debonding through com­plete separation and subsequent void growth within a unified framework. The formulation is a purely continuum one using a cohesive zone (Barenblatt, 1962; Dugdale, 1960) type model for the interface but with full account taken of finite geometry

A continuum model for void nucleation by inclusion debonding

The mechanisms of hydrogen-related fracture are briefly reviewed and a few evaluative statements are made about the stress-induced hydride formation, decohesion, and hydrogen-enhanced localized plasticity mechanisms. A more complete discussion of the failure mechanism based on hydrogen-enhanced dislocation mobility is presented, and these observations are related to measurements of the macroscopic flow stress. The effects of hydrogen-induced slip localization on the measured flow stress is discussed. A theory of hydrogen shielding of the interaction of dislocations with elastic stress centres is outlined. It is shown that this shielding effect can account for the observed hydrogen-enhanced dislocation mobility.

Hydrogen-enhanced localized plasticity—a mechanism for hydrogen-related fracture

Environmentally Assisted Cracking: Overview of Evidence for an Adsorption-Induced Localised-Slip Process,

The effect of hydrogen on dislocation dynamics

T he hydrogen transport problem is studied in conjunction with large deformation elastic—plastic behavior of a material. Oriani's equilibrium theory is used to relate the hydrogen in traps (micro-structural defects) to concentration in normal interstitial lattice sites (NILS). The resulting non-linear transient hydrogen diffusion equations are integrated using a modified backward Euler method. Coupled diffusion and plastic straining is analysed with this numerical procedure in the area around a blunting crack tip. A uniform NILS concentration as dictated by Sievert's law at the pressure and temperature of interest is used as initial condition throughout the body. The crack is initially blunted by plane strain mode I (tensile) loading. The finite element results show that hydrogen residing at NILS is generally very small in comparison with the population that develops in trapping sites near the crack surface. That is, lattice diffusion delivers the hydrogen but it is predominantly the trapping that determines its distribution at temperatures of interest. The predominance of trapped hydrogen over lattice concentration prevails even in the case when hydrogen migrates under steady state conditions. Hence, the hydrostatic stress effect is less important than traps created by plastic straining as far as the creation of high total hydrogen concentration is concerned. The trapping site locations and the temperature determine the amounts and locations of high hydrogen concentrations. Consequently, ahead of a blunting crack tip, the total hydrogen concentration and plastic strain diminish with distance from the crack tip whereas the hydrostatic stress rises. This would seem to have significant consequences for fractures induced by the presence of hydrogen.

Numerical analysis of hydrogen transport near a blunting crack tip

Hydrogen embrittlement and grain boundary fracture

The effect of hydrogen on the solid solution strengthening and softening of nickel

: In the present work we have attempted to study hydrogen-related fracture of nickel using a variety of experimental methods with the principle aim of establishing the mechanism of fracture. Fracture of nickel was studied in low pressure hydrogen gas and for specimens containing solute hydrogen using slow strain rate tension tests. both intergranular and transgranular fracture was observed on testing in hydrogen gas depending on the heattreatment and on the pressure of trace impurities such as S, Mn, and Mg. The intergranular fracture mode was correlated with the segregation of S to the grain boundaries as measured by Auger and SIMS techniques. Specimens containing solute hydrogen fractured in an intergranular mode for all heattreatments. Detailed examination of both the intergranular and transgranular fracture surfaces indicated that neither was a truly 'brittle' fracture mode but that both indicated a high degree of local ductility. The effect of hydrogen on the plastic properties of nickel was examined using stress strain techniques and in situ deformation in an environmental cell in the HVEM. It was shown that hydrogen decreases the flow stress of nickel. In situ environmental cell fracture studies in hydrogen gas in the HVEM showed that the fracture process resulted from locally enhanced deformation at the crack tip due to the presence of hydrogen. This was confirmed by in situ fracture studies in the SEM. (Author)

Hydrogen Effects in Nickel-Embrittlement or Enhanced Ductility.

The effects of hydrogen on the plastic deformation of nickel and nickel-carbon alloys were studied using plastic deformation techniques over a wide range of strain rates at about 300 K. The emphasis of the study was on the behavior at very low strains and low strain rates. Hydrogen was introduced as a solute element by quenching from a gaseous H2 atmosphere or by testing in a gaseous H2 atmosphere. The behavior of a number of different purities of nickel with hydrogen additions was examined. The most significant impurity element seemed to be C and this element was varied over a wide composition range by annealing in different atmospheres. Both solution softening and solution strengthening was observed depending on the amount of H in solution relative to the amount of C in solution. The nature of this solution softening effect is discussed.

This paper describes the relationship between intergranular and transgranular fracture modes of hydrogen embrittlement. The interaction of a transgranular crack, propagating under constant stress in a gaseous hydrogen environment of 40kPa with a grain boundary, is shown. Transgranular fracture is observed when the hydrogen distribution is enhanced at the tip of a notch because either that is the point at which hydrogen enters from the gas phase or concentrates from the solid solution due to the stress field. Intergranular fracture is alternately observed when there is a hydrogen concentration at the grain boundary higher than at other places in the solid. Higher hydrogen concentrations have been observed at grain boundaries in Ni-H alloys using a Secondary Ion Mass Spectrometer technique. The presence of segregated S at the grain boundaries was also shown to enhance the concentration of hydrogen there.

F. Heubaum

Papers

Hydrogen embrittlement and grain boundary fracture

The effect of hydrogen on the solid solution strengthening and softening of nickel

Hydrogen Effects in Nickel-Embrittlement or Enhanced Ductility.

The effect of hydrogen on the solid solution strengthening and softening of nickel

Hydrogen embrittlement and grain boundary fracture